1. Field of the Invention
The present invention relates to phosphors used in cathode ray tubes (CRTs) for liquid crystal displays, and, more particularly, to phosphors of lithium aluminum oxide activated with iron.
2. Description of Related Art
An existing far-red phosphor used in luminescent lamp manufacturing consists of a lithium aluminum oxide activated with iron (LiAlO.sub.2 :Fe). By "far-red" is meant that region of the electromagnetic spectrum between about 600 to 800 nm. In commercially available material, the iron concentration is about 0.005 mole; this material has been optimized for fluorescent lamps where near-IR output is useful for enhancing plant growth. This phosphor, when used in cathode ray tubes (CRTs), has a much longer decay time than can be tolerated for use in liquid crystal light valves employing o-silicon as the photosensitive material. In addition, the light output at reasonable drive levels is insufficient to operate the light valve. When greater drive levels are used, increasing the light output, the spot size becomes too large, resulting in resolution outside specification.
No other commercially available phosphor satisfies the requirements for liquid crystal light valve (LCLV) CRTs in terms of spectral energy distribution, decay time, efficiency, and particle size. While LiAlO.sub.2 :Fe meets the spectral range, the decay time (30 msec) and particle size (&gt;16 .mu.m) of the commercially available material limits its use in liquid crystal light valve applications.
Other phosphors have been investigated in an attempt to find a phosphor having the correct characteristics in regard to spectral emission, decay time, efficiency, and small particle size. Such other phosphors include aluminum oxide:Cr, cadmium sulfide:Ag, zinc cadmium sulfide:Ag, zinc phosphate:Mn, yttrium oxysulfide:Eu, yttrium aluminum oxide:Eu, among others. None have been found to have the exact attributes of the LiAlO.sub.2 :Fe material; the commercially available material is made specifically for luminescent lamps and is not suitable for CRT applications because of its long decay time.
A need remains for a suitable phosphor for LCLV CRTs.